Molecular sieves coated with non-oxide ceramics

ABSTRACT

There is provided a molecular sieve coated with a non-oxide ceramic. The molecular sieve may be a zeolite, such as ZSM-5, and the ceramic coating may be, e.g., boron nitride. The coated molecular sieve may be prepared by contacting the molecular sieve with a ceramic precursor material comprising a thermally decomposable material, such as polyborazylene, and thermally decomposing this thermally decomposable material. The coated molecular sieves may be used as organic conversion catalysts. The non-oxide ceramic coating may alter or enhance the shape-selective properties of the molecular sieve by providing a diffusion barrier to molecules.

BACKGROUND

There is provided a molecular sieve, such as a zeolite, coated with aceramic, such as a boron nitride. There is also provided a method formaking these materials. There is also provided a process for using thesematerials, e.g., as catalysts.

Zeolite materials, both natural and synthetic, have been demonstrated inthe past to have catalytic properties for various types of hydrocarbonconversion. Certain zeolitic materials are ordered, porous crystallinealuminosilicates having a definite crystalline structure as determinedby X-ray diffraction, within which there are a large number of smallercavities which may be interconnected by a number of still smallerchannels or pores. These cavities and pores are uniform in size within aspecific zeolitic material. Since the dimensions of these pores are suchas to accept for adsorption molecules of certain dimensions whilerejecting those of larger dimensions, these materials are included inthe class of materials known as "molecular sieves" and are utilized in avariety of ways to take advantage of these properties.

Such molecular sieves, both natural and synthetic, include a widevariety of positive ion-containing crystalline aluminosilicates. Thesealuminosilicates can be described as a rigid three-dimensional frameworkof SiO₄ and AlO₄ in which the tetrahedra are cross-linked by the sharingof oxygen atoms whereby the ratio of the total aluminum and siliconatoms to oxygen atoms is 1:2. The electrovalence of the tetrahedracontaining aluminum is balanced by the inclusion in the crystal of acation, for example an alkali metal or an alkaline earth metal cation.This can be expressed wherein the ratio of aluminum to the number ofvarious cations, such as Ca/₂, Sr/₂, Na, K or Li, is equal to unity. Onetype of cation may be exchanged either entirely or partially withanother type of cation utilizing ion exchange techniques in aconventional manner. By means of such cation exchange, it has beenpossible to vary the properties of a given aluminosilicate by suitableselection of the cation. The spaces between the tetrahedra are occupiedby molecules of water prior to dehydration.

Prior art techniques have resulted in the formation of a great varietyof synthetic zeolites. The zeolite have come to be designated by letteror other convenient symbols, as illustrated by zeolite A (U.S. Pat. No.2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat.No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4(U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886),zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No.3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), ZSM-35 (U.S. Pat.No. 4,016,245), ZSM-38 (U.S. Pat. No. 4,046,859), and zeolite ZSM-23(U.S. Pat. No. 4,076,842), merely to name a few.

The SiO₂ /Al₂ O₃ mole ratio of a given zeolite is often variable. Forexample, zeolite X can be synthesized with SiO₂ /Al₂ O₃ ratios of from 2to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit ofthe SiO₂ /Al₂ O₃ ratio is unbounded. ZSM-5 is one such example whereinthe SiO₂ Al₂ O₃ ratio is at least 5 and up to infinity. U.S. Pat. No.3,941,871 (U.S. Pat. No. Re. 29,948) discloses a porous crystallinesilicate made from a reaction mixture containing no deliberately addedalumina in the recipe and exhibiting the X-ray diffraction patterncharacteristics of ZSM-5 type zeolites. U.S. Pat. Nos. 4,061,724;4,073,865 and 4,104,294 describe crystalline silicates ororganosilicates of varying alumina and metal content.

A number of synthetic zeolites have been prepared which may be said tobe isostructural with naturally occurring zeolites. Zeolites ZSM-35 andZSM-38 are, for instance, ferrierite-type zeolites. Zeolite ZK-20 (U.S.Pat. No. 3,459,676) is described as being isostructural with thenaturally occurring zeolite levynite.

Although zeolites include materials containing silica and alumina, it isrecognized that the silica and alumina portions may be replaced in wholeor in part with other oxides. More particularly, GeO₂ is an artrecognized substitute for SiO₂ and B₂ O₃, Cr₂ O₃, Fe₂ O₃, and Ga₂ O₃ areart recognized replacements for Al₂ O₃. Accordingly, the term zeolite asused herein shall connote not only materials containing silicon and,optionally, aluminum atoms in the crystalline lattice structure thereof,but also materials which contain suitable replacement atoms for suchsilicon and/or aluminum. On the other hand, the term aluminosilicatezeolite as used herein shall define zeolite materials consistingessentially of silicon and, optionally, aluminum atoms in thecrystalline lattice structure thereof, as opposed to materials whichcontain substantial amounts of suitable replacement atoms for suchsilicon and/or aluminum.

The entire disclosures of the above-mentioned U.S. patents are expresslyincorporated herein by reference.

SUMMARY

According to an aspect of this application, there is provided amolecular sieve material coated with a non-oxide ceramic.

According to another aspect of this application, there is provided amethod for preparing a coated molecular sieve material, said methodcomprising the steps of:

(i) contacting a molecular sieve with a ceramic precursor material, saidceramic precursor material being capable of being converted into anon-oxide ceramic coating; and

(ii) converting the precursor material of step (i), whereby a non-oxideceramic coating is formed on said molecular sieve.

According to another aspect of this application, there is provided aprocess for converting an organic, said process comprising contactingsaid organic with a catalyst comprising a molecular sieve coated with anon-oxide ceramic.

EMBODIMENTS

Although molecular sieves are exemplified herein primarily as zeolites,it will be understood that the present invention pertains to other formsof molecular sieves including amorphous, as well as crystalline,materials. Such molecular sieves include activated carbon, amorphousaluminosilicates, aluminophosphates as described in U.S. Pat. Nos.4,310,440 and 4,385,994, silicoaluminophosphates (SAPOs) as described inU.S. Pat. No. 4,440,871, ELAPSOs as described in U.S. Pat. Nos.4,704,478 and 4,701,562, MeAPOs as described in U.S. Pat. No. 4,567,029,FeAPOs as described in U.S. Pat. No. 4,554,143, TAPOs as described inU.S. Pat. No. 4,500,651, FCAPOs as described in U.S. Pat. No. 4,686,093and layered materials as described in U.S. Pat. No. 4,859,648.

The molecular sieve may also be coated by contacting the molecular sievewith a material which is capable of thermally decomposing to form anon-oxide ceramic material, followed by heating the contacted materialto a temperature sufficient to decompose the thermally decomposablematerial. When the molecular sieve is coated in this manner, themolecular sieve should be a material which is thermally stable underthese heating conditions.

Various ceramic precursors are described in U.S. Pat. Nos. 4,801,439;4,810,436; 4.832,,895;and 4,833,103. A particluar ceramic precursorcomprises polyborazylene (B₃ N₃ H₄)_(x) which is described in thearticle by P. F. Fazen, J. S. Beck, A. T. Lynch, E. R. Remsen, and L. G.Sneddon, Chemistry of Materials 1990, Volume 2, pgs. 96-97. Ceramicprecursors may be either monomeric or polymeric thermally decomposablematerials, and these ceramic precursors may optionally comprise solventsfor the thermally decomposable materials.

The molecular sieve may also be coated by contacting the molecular sievewith a dispersion of a non-oxide ceramic, such as BN, in an organicmedia, followed by evaporation of the organic media. In this instancethe dispersion of the non-oxide ceramic in the organic media may beconsidered to be the ceramic precursor. This dispersion may be, forexample, a colloidal dispersion.

Non-oxide ceramics which may be coated on molecular sieves includeborides, carbides, nitrides, phosphides and silicides. Examples of suchceramics include BN, BNC, AlN, GaN, BP, B₄ C, TiB₂, Si₃ N₄ and SiC. Inaddition to polyborazylene, examples of thermally decomposable materialsin ceramic precursors include (CH₃)₂ S:BHBr₂, (CH₃)₃ N:AlH₃, NH₃ :BH₃,Al(BH₄)₃, (CH₃)₂ SiCl₂, and (CH₃)HSiCl₂.

In general, polymeric thermally decomposable materials includepolyvinylpentaborane, polyvinylborazine, polycarbosilanes, polysilanes,polycarbosiloxanes, polysilazanes and polycarborane siloxanes. Ingeneral, molecular thermally decomposable materials include aluminumamides, borazines, base-boranes, base alanes, base-gallanes and silanes.

In order to accomplish thermal decomposition, it may be necessary to usea decomposition gas during thermal treatment. For example, the molecularspecies (CH₃)₂ S:BHBr₂ will decompose to BN only if the decomposition iscarried out under ammonia. Examples of appropriate decomposition gases,which may be used with certain thermally decomposable materials, includeN₂, Ar, PH₃, NH₃ and AsH₃.

Provided that suitable decomposition gases are used, where necessary,the following polymeric thermally decomposable materials may beconverted into the following coatings (the composition of the coating isgiven in parentheses): polyvinylpentaborane (BN); polyvinylborazine(BN); polycarbosilanes (SiC); polysilanes (SiC); polycarbosiloxanes(SiC); polysilazanes (Si₃ N₄ /SiC); and polycarborane siloxanes (B₄C/SiC).

Prior to being contacted with the ceramic precursor, the molecular sievemay be optionally combined with a binder material which is permeable tothe ceramic precursor. Depending upon the relative sizes of themolecular sieve pores and the ceramic precursor, the ceramic coating mayform primarily on the exterior surface of the molecular sieve or thiscoating may extend into the interior pore space of the molecular sieve.Without wishing to be bound by any particular theory, it istheoretically possible that in some instances the ceramic coating isbound to the molecular sieve merely by physical interaction between thetwo species, while in other instances chemical interaction, e.g., interms of covalent bonding, may take place.

Particular examples of molecular sieves which may be coated withnon-oxide ceramics include zeolites having a Constraint Index of from 1to 12. ZSM-5 is an example of such a zeolite.

Molecular sieves include materials, such as zeolites, having ionexchange capacity. The original alkali metal cations of the assynthesized zeolite can be replaced in accordance with techniques wellknown in the art, at least in part, by ion exchange with other cations.Preferred replacing cations include metal ions, hydrogen ions, hydrogenprecursor, e.g. ammonium, ions and mixtures thereof. Particularlypreferred cations are those which render the zeolite catalyticallyactive, especially for hydrocarbon conversion. Replacing cations includehydrogen, rare earth metals and metals of Groups IA, IIA, IIIA, IVA, IB,IIB, IIIB, IV and VIII of the Periodic Table of the Elements.

A typical ion exchange technique would be to contact the syntheticzeolite with a salt of the desired replacing cation or cations. Examplesof such salts include the halides, e.g. chlorides, nitrates andsulfates.

Zeolites can be used either in the alkali metal form, e.g. the sodium orpotassium form; the ammonium form; the hydrogen form or anotherunivalent or multivalent cation form. When used as a catalyst thezeolite will be subjected to thermal treatment to remove part or all ofthe organic constituent.

The coated molecular sieves described herein can be used as a catalystin intimate combination with a hydrogenating component such as tungsten,vanadium, molybdenum, rhenium, nickel, cobalt, chromium, manganese, or anoble metal such as platinum or palladium where ahydrogenation-dehydrogenation function is to be performed. Suchcomponent can be exchanged into the composition to the extent atom Y,e.g., aluminum, is in the structure, impregnated therein or intimatelyphysically admixed therewith. Such component can be impregnated in or onto it such as for example, by, in the case of platinum, treating thecoated molecular sieve having ion exchange capacity with a solutioncontaining a platinum metal-containing ion. Thus, suitable platinumcompounds include chloroplatinic acid, platinous chloride and variouscompounds containing the platinum amine complex. The hydrogenatingcomponent may also be combined with the molecular sieve prior to theapplication of the coating of the non-oxide ceramic.

The coated molecular sieve of the present invention, when employedeither as an adsorbent or as a catalyst in an organic compoundconversion process should usually be dehydrated, at least partially.This can be done by heating to a temperature in the range of 200° C. to595° C. in an inert atmosphere, such as air, nitrogen, etc. and atatmospheric, subatomspheric or superatmospheric pressures for between 30minutes and 48 hours. Dehydration can also be performed at roomtemperature merely by placing the coated molecular sieve in a vacuum,but a longer time is required to obtain a sufficient amount ofdehydration.

The coated molecular sieve of the instant invention can be shaped into awide variety of particle sizes. Generally speaking, the particles can bein the form of a powder, a granule, or a molded product, such as anextrudate having particle size sufficient to pass through a 2 mesh(Tyler) screen and be retained on a 400 mesh (Tyler) screen. In caseswhere a catalyst is molded, such as by extrusion, the catalyst can beextruded before drying or partially dried and then extruded.

In the cases of many catalysts, it is desired to incorporate thecatalytic coated molecular sieve with another material resistant to thetemperatures and other conditions employed in organic conversionprocesses. Such materials inlcude active and inactive material andsynthetic or naturally occurring zeolites as well as inorganic materialssuch as clays, silica and/or metal oxides, e.g. alumina. The latter maybe either naturally occurring or in the form of gelatinous precipitatesor gels including mixtures of silica and metal oxides. Use of a materialin conjunction with the coated molecular sieve, i.e. combined therewith,which is active, tends to improve the conversion and/or selectivity ofthe catalyst in certain organic conversion processes. Inactive materialssuitably serve as diluents to control the amount of conversion in agiven process so that products can be obtained economically and orderlywithout employing other means for controlling the rate of reaction.These materials may be incorporated into naturally occurring clays, e.g.bentonite and kaolin, to improve the crush strength of the catalystunder commercial operating conditions. Said materials, i.e. clays,oxides, etc., function as binders for the catalyst. It is desirable toprovide a catalyst having good crush strength because in commercial useit is desirable to prevent the catalyst from breaking down intopowder-like materials. These clay binders have been employed normallyonly for the purpose of improving the crush strength of the catalyst.

Naturally occurring clays which can be composited with the materials ofthe present invention include the montmorillonite and kaolin familieswhich include the subbentonites, and the kaolins commonly known asDixie, McNamee, Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite, oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment or chemicalmodification. Binders useful for compositing with the present materialalso include inorganic oxides, notably alumina.

In addition to the foregoing materials, the coated molecular sieve ofthe present invention can be composited with a porous matrix materialsuch as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-beryllia, silica-titania as well as ternary compositions such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesiaand silica-magnesia-zirconia. The relative proportions of finely dividedcrystalline material and inorganic oxide gel matrix vary widely, withthe coated molecular sieve content ranging from about 1 to about 90percent by weight and more usually, particularly when the composite isprepared in the form of beads, in the range of about 2 to about 80weight percent of the composite.

The coated molecular sieve of the present invention when possessing acidactivity is useful as a catalyst component for a variety of organic,e.g. hydrocarbon, compound conversion processes. Such conversionprocesses include, as non-limiting examples, cracking hydrocarbons withreaction conditions including a temperature of from about 300° C. toabout 700° C., a pressure of from about 0.1 atmosphere (bar) to about 30atmospheres and a weight hourly space velocity of from about 0.1 toabout 20; dehydrogenating hydrocarbon compounds with reaction conditionsincluding a temperature of from about 300° C. to about 700° C., apressure of from about 0.1 atmosphere to about 10 atmospheres and aweight hourly space velocity of from about 0.1 to about 20; convertingparaffins to aromatics with reaction conditions including a temperatureof from about 100° C. to about 700° C., a pressure of from about 0.1atmosphere to about 60 atmospheres, a weight hourly space velocity offrom about 0.5 to about 400 and a hydrogen/hydrocarbon mole ratio offrom about 0 to about 20; converting olefins to aromatics, e.g. benzene,toluene and xylenes, with reaction conditions including a temperature offrom about 100° C. to about 700° C., a pressure of from about 0.1atmosphere to about 60 atmospheres, a weight hourly space velocity offrom about 0.5 to about 400 and a hydrogen/hydrocarbon mole ratio offrom about 0 to about 20; converting alcohols, e.g. methanol, or ethers,e.g. dimethylether, or mixtures thereof to hydrocarbons includingaromatics with reaction conditions including a temperature of from about300° C. to about 550° C., more preferably from about 370° C. to about500° C., a pressure of from about 0.01 psi to about 2000 psi, morepreferably from about 0.1 psi to about 500 psi, and a liquid hourlyspace velocity of from about 0.5 to about 100; isomerizing xylenefeedstock components with reaction conditions including a temperature offrom about 230° C. to about 510° C., a pressure of from about 3atmospheres to about 35 atmospheres, a weight hourly space velocity offrom about 0.1 to about 200 and a hydrogen/hydrocarbon mole ratio offrom about 0 to 100; disproportionating toluene with reaction conditionsincluding a temperature of from about 200° C. to about 760° C., apressure of from about atmospheric to about 60 atmospheres and a weighthourly space velocity of from about 0.08 to about 20; alkylatingaromatic hydrocarbons, e.g. benzene and alkylbenzenes, in the presenceof an alkylating agent, e.g. olefins, formaldehyde, alkyl halides andalcohols, with reaction conditions including a temperature of from about340° C. to about 500° C., a pressure of from about atmospheric to about200 atmospheres, a weight hourly space velocity of from about 2 to about2000 and an aromatic hydrocarbon/alkylating agent mole ratio of fromabout 1/1 to about 20/1; and transalkylating aromatic hydrocarbons inthe presence of polyalkylaromatic hydrocarbons with reaction conditionsincluding a temperature of from about 340° C. to about 500° C., apressure of from about atmospheric to about 200 atmospheres, a weighthourly space velocity of from about 10 to about 1000 and an aromatichydrocarbon/polyalkylaromatic hydrocarbon mole ratio of from about 1/1to about 16/1.

EXAMPLE

A solution of polyborazylene (0.50 g) in 20 ml of tetrahydrofuran wasadded to a beaker containing 2.00 g of calcined ZSM-5. It is noted thatthis ZSM-5 consisted essentially of relatively large crystallites of thetype described in U.S. Pat. No. 4,375,458. The contents were thenstirred for 0.5h. After removing the liquid phase by vacuum evaporation,the dried solids were transferred to a horizontal tube furnace. Thesample was heated under nitrogen at 2° C. per minute to a maximumtemperature of 600° C.

This coating process reduced the diffusion rate (D/r2) for this zeolitefrom 3.8×10⁻⁶ sec⁻¹ for the uncoated zeolite to 2.1×10⁻⁷ sec⁻¹ for thecoated zeolite. However, the ultimate benzene sorption for both thecoated and uncoated zeolites was 7 wt. %. Therefore, the coating processestablished a diffusion barrier without occluding the zeolitic porestructure.

When both of the coated and uncoated zeolites were used as catalysts todisproportionate toluene, the coated zeolite produced a greaterpercentage of the paraisomer among the xylenes produced.

What is claimed is:
 1. A zeolite material coated with a non-oxideceramic, wherein said non-oxide ceramic is bound to the surface of saidzeolite.
 2. A material according to claim 1, wherein said non-oxideceramic is selected from the group consisting of borides, carbides,nitrides, phosphides and silicides.
 3. A material according to claim 1,wherein said non-oxide ceramic is selected from the group consisting ofBN, BNC, AlN, GaN, BP, B₄ C, TiB₂, Si and SiC.
 4. A material accordingto claim 1, wherein said non-oxide ceramic is BN.
 5. A materialaccording to claim 1, wherein said zeolite has a Constraint Index from 1to
 12. 6. A material according to claim 1, wherein said zeolite isZSM-5.
 7. A material according to claim 6, wherein said non-oxideceramic is BN.
 8. A method for preparing a coated zeolite material, saidmethod comprising the steps of:(i) contacting a zeolite with a ceramicprecursor material, said ceramic precursor material being capable ofbeing converted into a non-oxide ceramic coating; and (ii) convertingthe precursor material of step (i), whereby a non-oxide ceramic coatingis formed on said zeolite, said non-oxide ceramic coating being bound tothe surface of said zeolite.
 9. A method according to claim 8, whereinsaid ceramic precursor is a dispersion of a non-oxide ceramic in anorganic media and step (ii) comprises evaporating said organic media toform said non-oxide coating.
 10. A method according to claim 8, whereinsaid ceramic precursor material comprises a material which is capable ofthermally decomposing to form a non-oxide ceramic material and step (ii)comprises heating the material of step (i) to a temperature sufficientto decompose said material which is capable of thermally decomposing.11. A method according to claim 9 wherein said dispersion is a colloidalof BN.
 12. A method according to claim 10, wherein said material whichis capable of thermally decomposing is a monomeric material, themolecules thereof being too large to diffuse into the interior porespace of said zeolite.
 13. A method according to claim 10, wherein saidmaterial which is capable of thermally decomposing is a polymericmaterial, the polymers thereof being too large to diffuse into theinterior pore space of zeolite.
 14. A method according to claim 10,wherein said ceramic precursor material comprises a material selectedfrom the groupconsisting of polyborazylene, (CH₃)₂ S:BHBr₂, (CH₃)₃N:AlH₃, NH₃ :BH₃, Al(BH₄)₃, and (CH₃)₂ SiCl₂.
 15. A method according toclaim 10, wherein said ceramic precursor material comprisespolyborazylene.
 16. A method according to claim 10, wherein said zeolitehas a Constraint Index from 1 to
 12. 17. A method according to claim 16,wherein said zeolite is ZSM-5.
 18. A method according to claim 17,wherein said ceramic precursor material comprises a solutionpolyborazylene in tetrahydrofuran.